Stereoselective Iridium-N,P-Catalyzed Double Hydrogenation of Conjugated Enones to Saturated Alcohols

Consecutive Asymmetric Transfer Hydrogenation of C2-Acylated Quinolines and Quinoxalines: A Diastereodivergent Synthesis of Enantioenriched Tetrahydroquinolines and Tetrahydroquinoxalines Bearing Endo- and Exocyclic Chirality

Consecutive asymmetric hydrogenation offers a direct and convenient approach to synthesizing complex C(sp3)-enriched products with multiple chirality. Herein, we report an asymmetric synthesis of chiral 1,2,3,4-tetrahydroquinolines (THQs) and tetrahydroquinoxalines bearing both endo- and exocyclic vicinal chirality through the consecutive transfer hydrogenation of easily accessible C2-acylated quinolines and quinoxalines. The method features mild conditions, easy operation, broad substrate scope (42 examples), and excellent asymmetric control (generally >90% ee and 20/1 dr). The key to success is the use of a water-soluble chiral aminobenzimidazole Ir catalyst. Mechanistic experiments support that the reaction involves the sequential reduction of the carbonyl group and then the quinoline core, with the asymmetric control of each step dominated by the catalyst. Remarkably, a diastereodivergent synthesis of all four stereoisomers of a chiral THQ has been successfully implemented.

Stereocontrolled Hydrogenation of Conjugated Enones to Alcohols via Dual Iridium-Catalysis

The concept of dual catalysis is an emerging area holding high potential in terms of preparative efficiency, yet faces severe challenges in compatibility of reaction conditions and interference of catalysts. The transition-metal catalyzed stereoselective hydrogenation of olefins and ketones typically proceeds under different reaction conditions and/or uses a different reductant. As a result, these two types of hydrogenations can normally not be performed in the same pot. Herein, the stereocontrolled hydrogenation of enones to saturated alcohols is described, enabled by orthogonal dual iridium catalysis, using molecular hydrogen for both reductions. In this one-pot procedure, N,P-iridium catalysts (hydrogenation active towards olefins) and NHC,P-iridium catalysts (hydrogenation active towards ketones) operated independently of one another allowing the construction of two contiguous stereogenic centers up to 99 % ee, 99/1 d.r. Ultimately, by simple selection of the chirality of either ligands, the enone could be efficiently reduced to all four stereoisomers of the saturated alcohol in equally high stereopurity. This degree of stereocontrol for the synthesis of different stereoisomers by dual transition-metal catalyzed hydrogenation was previously not attained. The generality in substituted enones (alkyl, aryl, heteroaryl) demonstrate the wide applicability of this concept.

Asymmetric and Chemoselective Iridium Catalyzed Hydrogenation of Conjugated Unsaturated Oxime Ethers

Research on the chemoselective metal-catalyzed hydrogenation of conjugated π-systems has mostly been focussed on enones. Herein, we communicate the understudied asymmetric hydrogenation of enimines catalyzed by N,P-iridium complexes and chemoselective toward the alkene. A number of enoxime ethers underwent hydrogenation smoothly to yield the desired products in high yield and stereopurity (up to 99 % yield, up to 99 % ee). No hydrogenation of the C=N π-bond was observed under the applied reaction conditions (20 bar H2, rt, DCM). It was demonstrated that the chiral oxime ether could be hydrolyzed into the ketone with complete preservation of the installed stereogenity at the α-carbon. At last, a binding mode of the substrate to the active iridium catalyst and the consequence for the stereoselective outcome was proposed.

Ru-catalyzed asymmetric hydrogenation of α,β-unsaturated ketones via a hydrogenation/isomerization cascade

Ru-catalyzed asymmetric hydrogenation of α-substituted α,β-unsaturated ketones has been developed for the enantioselective synthesis of chiral α-substituted secondary alcohols with high diastereo- and enantioselectivities (up to >99 : 1 dr, 98% ee). Mechanistic experiments suggest that the reaction proceeds via a Ru-catalyzed asymmetric hydrogenation of the CO bond in concert with a base-promoted allylic alcohol isomerization, and the final stereoselectivities were controlled by a DKR process during the asymmetric hydrogenation of the ketone intermediate.

Co-Promoted CoNi Bimetallic Nanocatalyst for the Highly Efficient Catalytic Hydrogenation of Olefins

Bimetallic catalysts, especially non-noble metals, hold great potential for substituting for noble metals in catalytic hydrogenation. In present study, a series of Co_xNi_y (x + y = 6) bimetallic catalysts were prepared through the impregnation-reduction method and cyclohexene was chosen as probe-molecule to study the promotion effect of Co on the catalytic olefin hydrogenation reactions. Meanwhile, density functional theory (DFT) was utilized to investigate the formation energies and the charge distribution of CoNi bimetals, as well as the transition state (TS) searches for hydrogen dissociation and migration. The results suggest that bimetals tend to have superior catalytic performance than pure metals, and Co₃Ni₃ shows the highest catalytic activity on the cyclohexene hydrogenation. It was found that the charge transfer from Co to Ni and the alloying give rise to the refinement of CoNi grains and the improvement of its catalytic activity and stability. Thus, it may be possible to obtain better catalytic performance by tuning the metal/metal atomic ratio of bimetals.

Enantiodivergent Hydrogenation of Exocyclic α,β-Unsaturated Lactams Enabled by Switching the N-Chirality of Iridium Catalyst

Central chirality is an important chiral element used in the design of chiral ligands and catalysts. Mostly, the attention of organic chemists is focused on developing of chiral ligands with stable stereogenic centers. However, the N-chirality in chiral ligand design has been rarely explored due to its flexibility. Here we demonstrate the design, synthesis, and application of a class of simple P,N-ligands with flexible N-chirality and their derived iridium complexes with fixed N-chiral stereocenters. Both fixed configurations of the N-stereocenter of the iridium complexes could be selectively formed from the same chiral ligand. This pair of diastereoisomeric iridium complexes showed good performance in the enantiodivergent asymmetric hydrogenation of exocyclic α,β-unsaturated lactams. The N-H group plays an impressive role in catalytic activity. Computational studies emphasized the importance of N-chirality and N-H group.

Iridium-Catalyzed Double Asymmetric Hydrogenation of 2,5-Dialkylienecyclopentanones for the Synthesis of Chiral Cyclopentanones

Herein, we report an efficient iridium-catalyzed double asymmetric hydrogenation of 2,5-dialkylienecyclopentanones, delivering the chiral 2,5-disubstituted cyclopentanones in excellent yields and stereoselectivities. The results of the kinetic experiments and control experiments indicated that the two C═C bonds were hydrogenated in a stepwise manner and the second stereocenter was synergistically controlled by the chiral catalyst and the chirality of monohydrogenated product. The hydrogenated products can be prepared on a gram-scale and are easily derivatized.

Rh-Catalyzed Sequential Asymmetric Hydrogenations of 3-Amino-4-Chromones Via an Unusual Dynamic Kinetic Resolution Process

Rh-catalyzed sequential asymmetric hydrogenations of 3-amino-4-chromones have been achieved for the first time via an unprecedented dynamic kinetic resolution under neutral conditions, providing (S,R)-3-amino-4-chromanols in high yields (up to 98%) with excellent enantio- and diastereoselectivities (up to 99.9% ee and 20:1 dr). The mechanistic studies based on control experiments and density functional theory (DFT) calculations suggest that the dynamic kinetic resolution process for the intermediate enantiomers generated in the first hydrogenation step proceeded via a stereomutation (or called chiral assimilation) pathway from an undesired enantiomer to the desired enantiomer rather than via traditional racemization of the undesired enantiomer. The protocol can be performed on a gram scale with a relatively low catalyst loading and offers a practical and convenient pathway for synthesizing a series of bioactive chromanols and their derivatives.

Ag2O/squaramide cocatalyzed asymmetric interrupted Barton-Zard reaction of 8-nitroimidazo[1,2-a]pyridines

Imidazo[1,2-a]pyridines are present in numerous biologically active compounds as the core structural motif. Herein, we report an asymmetric interrupted Barton-Zard reaction of electron-deficient imidazo[1,2-a]pyridines with α-substituted isocyanoacetates. The reaction enables the dearomatization of 8-nitroimidazo[1,2-a]pyridines and hence offers straightforward access to an array of optically active highly functionalized imidazo[1,2-a]pyridine derivatives that possess three contiguous stereogenic centers in good yields (up to 98%) with high stereoselectivities (>19:1 dr, >99% ee). It is worth noting that the catalytic system consisting of a chiral squaramide and silver oxide displays remarkable reactivity and stereoselectivity, and a gram-scale reaction is compatible with the catalyst loading of 0.5 mol%. In addition, the synthetic potential of this method was showcased by versatile transformations of the product.